GSoC 2018: SymPy - Week 6 & 7

The second phase of Coding Period has started.

I started this phase working on recurrence submodule under discrete. After having an initial discussion with Kalevi regarding the functionality to be implemented, I did the proof-of-concept for the same on a remote branch.

After the approach was finalized, the implementation was polished before opening the PR. The PR #14816 also included documentation, doctests, and unit tests for the module.